Cellular communication systems using spread spectrum encoding are known. Direct sequence (DS) and frequency hopping are the two most well known of the code division multiple access (CDMA) spread spectrum technologies in which an information signal is distributed over a relatively wide spectral area for purposes of reducing the impact of interference. Frequency hopping achieves the benefits of spread spectrum transmission by hopping through a number of conventional narrowband channels, thereby reducing the average impact of interference in any one channel or group of channels.
DS-CDMA spreads an information signal over a designated spectrum by modulating the information signal with a spreading code having properties which ideally would be random in nature. The spreading code actually used, on the other hand, is typically the output of a pseudorandom number generator which provides a number sequence that repeats over a relatively long time interval.
At a receiver the DS spread spectrum signal must be de-spread through use of a de-spreading code that has the same characteristics as the spreading signal. De-spreading is accomplished by the correlation of the received spread signal with a synchronized replica of the spreading code.
The use of a spreading code that has the characteristics of a random signal (and a random distribution of included frequencies) ensures that the transmitted signal with be randomly spread throughout a transmission spectrum. The random distribution of frequencies is ensured by the random nature of the signal itself and the fact that an interaction of a relatively large number of frequencies is necessary to cause the random path followed by a random signal.
The requirement for a duplicate of the spreading signal at a receiver (for de-spreading) requires that the spreading (and de-spreading) code be a repeating sequence known to both transmitter and receiver. The use of identical spreading and de-spreading codes at both transmitter and receiver provides the basis for communication between multiple pairs of communicating parties within the same spectrum under the DS-CDMA format.
Service for DS-CDMA communication units is typically provided through a base transceiver station (BTS) providing communication service within an associated geographical area. Since a number of communiction units may require simultaneous service within the geographic area, the BTS must be capable of simultaneously transceiving numerous CDMA signals. While a number of CDMA transceivers and antenna could be located at the BTS to service the signals, such an approach would be prohibitively expensive. A more economical approach has been to transceive the signals through a single antenna using transceivers suited for such use.
To accomplish the task of transceiving multiple CDMA signals through a common antenna, an output of each active CDMA channel is typically combined before power amplification. Power amplification of the multitude of signals transmitted through the common antenna is accomplished through the use of a linear power amplifier (LPA).
The prior art has taught that one method of combining CDMA signals is by, first, converting each digital CDMA signal sample into its analog equivalent and, then, combining the multitude of signals within a resistive array. While such an approach is effective, it is difficult to implement in a CDMA system. One problem relates to synchronization of a number of CDMA signals to a common reference. The communications regulatory body on CDMA (Electronic Industry Association/Telecommunications Industry Association (EIA/TIA) standards Interim Standard (IS)95) has mandated that each CDMA signal of the multitude of signals transmitted by a BTS shall be synchronized to within plus or minus 50 ns of a pilot signal for each channel. Conversion of CDMA signals to an analog format and summing within a resistive array introduces indeterminant time delays which are inconsistent with the EIA/TIA standards and the need for precise synchronization among CDMA signals.
Another problem inherent in the use of a resistive array for combining CDMA signals lies in the difficulty in trouble-shooting such a system. Where a problem occurs, a resistive array allows signals to travel both upstream and downstream within the array making it difficult to isolate individual signals for analysis.
While the prior art use of resistive arrays combining techniques has proven effective in some applications, such use is expensive and often difficult to implement where synchronization requirements are precise. A need exists for a method of combining CDMA signals that facilitates synchronization among the combined signals, channel maintenance and failure analysis.